Week 26

CST-334 Week 2

This week was our second week in CST-334, also known as Operating Systems.

Operating Systems: Three Easy Pieces

This week we read chapters 4-8 of our course textbook.

Chapter 4: Processes

This chapter discusses processes, which are running programs, and how the OS virtualizes the CPU to run multiple processes.

Key Takeaways:

  • The OS runs multiple processes by stopping and starting them during context switches.
  • All OSs have APIs that processes use to interact with resources and the OS.
  • To create a process executable code of a program must be loaded from storage onto memory.
  • Processes can be running, ready, or blocked.
  • The OS tracks data for each process. 

Chapter 5: Process API

This chapter discusses certain important aspects in Process APIs.

Key Takeaways:

  • The fork() system call creates a new process. The new process is identical to its parent process when it made the system call.
  • The wait() system call allows a process to wait for another process, or its child processes, to finish before continuing to run.
  • The exec() system call changes one process into another process. 

Chapter 6: Direct Execution

This chapter discusses how processes are ran on the CPU.

Key Takeaways

  • Processes run directly on the CPU. This means that the OS cannot run while a process is running.
  • The user mode, which processes typically run on, places certain restrictions on what a process can do.
  • The kernel mode, which the OS runs in, has no restrictions on what a process can do.
  • System calls allow processes to make restricted operations in a controlled, safe way.
  •  Processes are able to voluntarily give control back to the OS, such as when making system calls.
  • The OS is able to directly take control of the CPU through interrupts, such as the timer interrupt. 

Chapter 7: CPU Scheduling

This chapter discusses CPU and process scheduling.

Key Takeaways:

  • Turnaround time measures how quickly a job completed by comparing its arrival and completion time.
  • Response time measures how quickly a job began work by comparing its arrival time and the time it first began running.
  • First In, First Out (FIFO) completes jobs in order of when they arrived.
  • Shortest Job First (SJF) completes jobs based on their total duration.
  • Shortest Time-to-Completion First (STCF) completes jobs based on their time remaining.
  • Round Robin (RR) Switches between each process regularly to improve response time.
  • When a process makes a call for I/O it is blocked until the I/O request is completed, allowing other processes to run. 

Chapter 8: Multi-Level Feedback

This chapter discusses Multi-level Feedback Queues (MLFQ).

Key Takeaways:

  • MLFQs sort jobs by priority.
  • Jobs with higher priority are run before those with lower priority.
  • Jobs with the same priority are run with the Round Robin schedule.
  • When a job arrives it is placed at the highest priority.
  • When a job runs for an allotted amount of time it drops in priority.
  • Priority boosts occasionally boost all jobs to the highest priority, ensuring that no job starves, and that jobs that change behavior are able to change priority.
   

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